Door latch assemblies are frequently made with a backset adjustment. The two standard backset dimensions for doors are 2-3/8 inches and 2-3/4 inches. A latch assembly which is suitable for both backset dimensions, avoids the necessity of retailers stocking different latch assemblies. Furthermore, purchasers are relieved of the requirement to measure the backset before purchasing a latch assembly. In the past a latch assembly with a backset adjustment had an arrangement wherein the rotational spindle for the handles could be located at one of two positions in the casing assembly. One such example of this arrangement is shown in U.S. Pat. No. 4,921,290 which shows two spindle positions in the latch assembly. The spindle may be moved between these two positions to adjust the backset.
Another example of a latch assembly with a backset adjustment is shown in U.S. Pat. No. 4,664,433. This patent discloses a latch assembly which includes a bolt assembly having two parts and a casing assembly in two parts. The forward or front bolt part and front case part rotate and slide one within the other through a half revolution. The same helical longitudinal movement of both the front bolt part and front case part is required to change the backset. In this patent a rotatable front casing has a helical groove therein with a protrusion on a non-rotatable back casing engaging this helical groove. A hollow forward bolt part is shown into which an integral hollow bolt extension and bolt adjuster is received. A pin extends through the hollow forward bolt part to engage a helical slot in the bolt adjuster. Thus, when the bolt part rotates with the front casing, both move outward or inward in the same helical movement to change the backset.
With the bolt part and the casing part moving along the same helical path they ideally retain the same relative position as they move from one backset position to another. Flats in the two extreme positions of the casing grooves cause the bolt to commence helical movement either slightly ahead or slightly behind the helical casing movement at the very beginning of the movement from one backset position to the other to prevent misalignment which could cause the bolt to fall out of the thin opening in the face plate and jam. However, due to tolerances between the moving parts it still is possible for the bolt part to fall behind the casing part after simultaneous helical movement has begun which in turn can result in the bolt part falling out of the face plate and jamming.
Another problem with this arrangement is space between the cylindrical portion of the bolt adjuster and the non-cylindrical hollow interior of the bolt is sufficient to allow considerable undesirable lateral movement of the bolt adjuster within the hollow interior of the bolt part. Consequently, the undesirable lateral movement of the bolt part can cause deflection which in some cases may result in jamming of the bolt, the latch mechanism becoming stiff or the bolt not engaging the strike plate in the door frame.